Almost 100 years now after discovering the bacillus Shiga, commonly known as Shigella dysenteriae, type 1, shigellosis is the one of the most important public health problems of almost all countries in the world. Annually, several hundred thousand children under the age of 5 die in developing countries from shigellosis caused by microorganisms of the genus Shigella. Outbreaks of shigellosis are occasionally registered in developing countries of the northern hemisphere, caused by the bacteria S. sonnei, the only representative of group D, genus Shigella. 
Relating to the aforementioned, WHO recommends as a priority goal the development of a “global” anti-shigella vaccine, including protective compounds for pathogenic bacteria of genus Shigella, specifically S. sonnei, phase I (Kotloff K. L., Winickoff J. P, Ivanoff B., Clemens J. D., Swerdlow D. L., Sansonetti P. J., Adak G. K., Levine M. M. Global burden of Shigella infections: implications for vaccine development and implementation of control strategies. Bull. WHO, 1999, v. 77, p. 651-665). Development of a monovaccine against shigellosis S. sonnei may be considered as a preliminary step for the solution of this general problem and as an independent project extremely actual for many regions.
The specificity of immunity to Shigella infection is determined by the structure of the Shigella's main protective antigen—the polysaccharide O-antigen. The primary structure of O-specific polysaccharide obtained from the lipopolysaccharide (LPS) molecule of cell walls of S. sonnei, phase I is identified by Kenne et al. (Kenne L., Lindberg B., Petersson K., Katzenellenbogen E., Romanowska E. Structural studies of the O-specific side-chains of the Shigella sonnei phase I lipopolysaccharide. Carbohydrate Res., 1980, 78:119-126).
The O-antigen component of LPS is a polysaccharide composed of repeating disaccharide units of O-[4-amino-2-(N-acetyl)amino-2,4-dideoxy-β-D-galactopyranosyl]-(1→4)-[2-(N-acetyl)amino-2-deoxy-α-L-altrpyranuronic acid] linked by (1→3) bonds to form a polysaccharide chain. This O-polysaccharide component of S. sonnei, phase I, covalently links to E. coli R2 type core domain, which, in turn, covalently links to lipid A forming a linear molecule LPS.
Isolation of O-polysaccharide from the cell wall LPS does not represent significant technical difficulties. Thus, the method of isolation, first proposed by Freeman, includes the following main stages—obtaining a culture of bacteria S. sonnei, phase I in liquid medium; separation of culture fluid from bacterial cells, extracting LPS from bacterial cell with aqueous phenol (Westphal O., Jann K. Bacterial lipopolysaccharide extraction with phenol: water and further application of the procedure. Methods Carbohydr. Chem., 1965, v. 5, p. 83-91); and degradation of LPS with further isolation of the O-polysaccharide from it (Morrison D. C., Leive L. Fractions of lipopolysaccharide from Escherichia coli O111:B4 prepared by two extraction procedures. J. Biol. Chem. 250 (1975) 2911-2919).
Another method of obtaining highly purified O-specific antigen of Shigella sp is also known and includes the following stages: obtaining bacterial cultures in liquid medium; treatment of bacterial cultures with hexadecyltrimethylammonium bromide and subsequent extraction of LPS from bacterial cells; separation of LPS extract from bacterial cells; and degradation of LPS with subsequent separation of O-polysaccharide from it (KR 20010054032 A). Thereby, all known methods of isolating O-specific antigens from Shigella sp. LPS are based on the stage of extraction, i.e. LPS extraction from bacterial cell walls, which causes the unavoidable loss of bacterial cell nativity.
It should be additionally noted, that the structure of O-specific antigens obtained by known methods from LPS's is determined by the genomes of Shigella sp bacteria.
Practically all O-antigens obtained from Shigella sp. LPS's contain elements of core domain structures. Mild hydrolysis using 1% acetic acid, which is used for removal of lipid A from the LPS molecule, leads to obtaining a polysaccharide derivative, which is represented as an O-specific polysaccharide, connected to the “core” oligosaccharide (Fensom and Meadow 1970; Morrison and Leive, 1975; Oertelt et al., 2001; Osborn, 1963.
It was proposed to use the O-polysaccharide from the LPS of the bacterial cell wall of S. sonnei, phase I, as a component of only conjugated vaccines against S. sonnei shigellosis, under its covalent bonding with protein carriers—protein D Haemophilis influenzae, recombinant exoprotein A Pseudomonas aeruginosa (rEPA), recombinant diphtheria toxin (rDT), recombinant toxin B Clostridium difficle (rBRU) (US Pat. Appl. 2005/0031646; WO/2010/019890).
Investigations were conducted of the immunogenic and protective properties of conjugates containing O-polysaccharide from the LPS of the bacterial cell walls of Plesiomonas shigelloides O7, whose structure is identical to O-polysaccharide from LPS of bacteria S. sonnei, phase I, and conjugated with protein—exoprotein A P. aeruginosa (rEPA) or diphtheria toxoid CRM9 from mutant strain Corynebacterium diphtheriae (Cohen D., Ashkenazi S., Green M. S., Gdalevich M., Robin G., Slepon R., Yavzori M., Orr N., Block C., Ashkenazi I., Shemer J., Taylor D. N., Hale T. L., Sadoff J. C., Pavliakova D., Schneerson R., Robbins R. Double-blind vaccine controlled randomized efficacy trial of an investigational Shigella sonnei conjugate vaccine in young adults. Lancet, 1997, v. 349, pp. 155-159). It has been found that the conjugate of O-polysaccharide with rEPA was immunogenic for experimental animals and humans when administered parenterally, causing in volunteers O-specific antibody production and average level of protection against infection with an efficacy coefficient of 74%. However, the rather short duration of the controlled experiment (2.5-7 months) is causing certain doubts in the rating for the protective potential of the vaccine. Recent immunogenicity trials on children of O-polysaccharide conjugate vaccine against S. sonnei infection based on rEPA-carrier revealed low immunogenicity of the preparation for children of ages from 1 to 4 years (efficacy coefficient was 27.5%), as well as the early declining of immune response after immunization (Passwell J H, Ashkenzi S, Banet-Levi Y, Ramon-Saraf R, Farzam N, Lerner-Geva L, Even-Nir H, Yerushalmi B, Chu C, Shiloach J, Robbins J B, Schneerson R; Israeli Shigella Study Group. Age-related efficacy of Shigella O-specific polysaccharide conjugates in 1-4-year-old Israeli children. Vaccine. 2010, March, 2; 28(10), pp. 2231-2235).
Thus, the protein-polysaccharide conjugate vaccines against shigellosis S. sonnei have shown an insufficient immunogenicity in clinical trials on adults and children. It should be noted that the immunogenic properties of free, unconjugated O-polysaccharide from the LPS of the S. sonnei bacteria, phase I, as a vaccine immunogen is not known. Experimental data from Taylor et al show a practically full absence of immunogenic activity in mice against unconjugated polysaccharide from LPS of bacterial cells Plesiomonas shigelloides, the structure of which is identical to that of S. sonnei, phase I O-antigen (Taylor D. N., Trofa A. C., Sadoff J., Chu C., Bryla D., Shiloach J., Cohen D., Ashkenazi S., Lerman Y., Egan W. Synthesis, characterization and clinical evaluation of conjugate vaccines composed of the O-specific polysaccharides of Shigella dysenteriae type 1, Shigella flexneri type 2a, and Shigella sonnei (Plesiomonas shigelloides) bound to bacterial toxoids. Infect. Immun., 1993, September, 61(9): 3678-3687).
Based on the aforementioned, the actuality of development of other approaches to the creation of O-antigen vaccines against S. sonnei infection is obvious. An alternative, and more promising approach for development can be considered: the creation of an unconjugated vaccine based on the O-antigen exopolysaccharide, produced by S. sonnei, phase I bacteria into the cultural medium. It is known, that many gram-positive and gram-negative bacteria produce not only polysaccharide components of cell walls, but also extracellular exopolysaccharides, which are secreted by the cell into the external medium and provide the protective function. Thus, the produced exopolysaccharides can be found both in a free state or form an extracellular capsule or microcapsule.
Sometimes exopolysaccharides produced by cells into the external medium represent specific highly-immunogenic antigens—capable of inducing protective antibody synthesis. Thus, a variety of such polysaccharide antigens are used in the vaccine compositions for prevention of infections, caused by meningococcus groups A and C, cause of typhoid fever (Lindberg A. A. Polyosides (encapsulated bacteria). C. R. Acad. Sci. Paris, 1999, v. 322, p. 925-932).
Polysaccharide vaccine immunogenicity is dependent on the primary structure of the polysaccharide antigen, its molecular mass, and ability to form aggregate structures (The vaccine book. Edited by B. R. Bloom, P.-H. Lambert Academic Press, San Diego 2003, pp. 436). At the same time, the primary structure of bacterial exopolysaccharide can be identical to or different from that of O-specific polysaccharide from the cell wall LPS. (Goldman R. C., White D., Orskov F., Orskov I., Rick P. D., Lewis M. S., Bhattacharjee A. K., Leive L. A surface polysaccharide of Esherichia coli O111 contains O-antigen and inhibits agglutination of cells by anti-O antiserum. J. Bacteriol., 1982, v. 151, p. 1210-1221).
However, neither the primary structure of the exopolysaccharide of bacteria S. sonnei, phase I, nor its physico-chemical, immunobiological, and protective properties, nor the method of its isolation, nor even the fact of its existence are described in the literature.
The literature also does not describe the pharmaceutical compositions based on S. sonnei, phase I polysaccharides, the development of which can make significant contributions to clinical pharmacology. Only known is the usage of fragments of polysaccharides from LPS of S. sonnei, phase I cells, including from 1 to 5 disaccharide units, as a nutrient supplement for oral administration, stimulating immune system development in infants between 1 and 6 months of age, determined by the increase of ratio of type 1 T-helpers (Th1 response) in relation to the type 2 T-helpers (Th2 response) ratio (US Pat. Appl. 2009/0317427 A1).